Re: aconnect occasionally causes kernel oopses

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On Tue, 03 Aug 2021 09:40:50 +0200,
folkert wrote:
> 
> Hi,
> 
> To which kernel version should I apply it?
> Because:
> 
> folkert@oensoens:~/linux-5.11.0$ patch -p1 < ~/alsa-patch.diff 
> patching file sound/core/seq/seq_ports.c
> patching file sound/core/seq/seq_ports.h
> patching file sound/core/seq/seq_ports.c
> Hunk #1 succeeded at 526 (offset 12 lines).
> Hunk #2 succeeded at 538 (offset 12 lines).
> Hunk #3 succeeded at 547 with fuzz 2 (offset 12 lines).
> Hunk #4 FAILED at 602.
> 1 out of 4 hunks FAILED -- saving rejects to file sound/core/seq/seq_ports.c.rej

Did you scratch the previous patch?  AFAIK, the latest patch should be
applicable to 5.11 and older cleanly.


Takashi

> 
> On Tue, Aug 03, 2021 at 08:55:36AM +0200, Takashi Iwai wrote:
> > On Mon, 02 Aug 2021 21:53:49 +0200,
> > folkert wrote:
> > > 
> > > > > [ kernel bug if repeatingly aconnect'ing midi devices ]
> > > > > 
> > > > > > Does this happen if you do reconnect of kernel sequencer client?
> > > > > > You can use snd-virmidi as well as snd-dummy.
> > > > > > I'm asking it because it'll simplify the test a lot, which will be
> > > > > > almost self-contained.
> > > > > 
> > > > > Like this?
> > > > > 
> > > > > root@lappiemctopface:~# aplaymidi -l
> > > > >  Port    Client name                      Port name
> > > > >  14:0    Midi Through                     Midi Through Port-0
> > > > >  20:0    Virtual Raw MIDI 1-0             VirMIDI 1-0
> > > > >  21:0    Virtual Raw MIDI 1-1             VirMIDI 1-1
> > > > >  22:0    Virtual Raw MIDI 1-2             VirMIDI 1-2
> > > > >  23:0    Virtual Raw MIDI 1-3             VirMIDI 1-3
> > > > > 128:0    rtpmidi lappiemctopface          Network
> > > > > 128:1    rtpmidi lappiemctopface          metronoom
> > > > > 128:2    rtpmidi lappiemctopface          AppleMidi2IPMidiBridge
> > > > > 128:3    rtpmidi lappiemctopface          oensoens
> > > > > 130:0    FLUID Synth (11462)              Synth input port (11462:0)
> > > > > 
> > > > > and then:
> > > > > 
> > > > > root@lappiemctopface:~# cat test.sh 
> > > > > while true
> > > > > do
> > > > > 	aconnect 20:0 21:0
> > > > > 	aconnect -d 20:0 21:0
> > > > > done
> > > > > 
> > > > > root@lappiemctopface:~# for i in `seq 0 3` ; do (./test.sh &) ; done
> > > > > 
> > > > > This hard locks-up my laptop: it doesn't even respond to capslock (led
> > > > > on/off) anymore nor the ctrl+prtscr+alt+b combination.
> > > > 
> > > > Thanks, that's really helpful!
> > > > I see the possible race now.
> > > 
> > > > Could you try the quick fix below?
> > > 
> > > Something may have changed:
> > > 
> > > folkert@oensoens:~$ aplaymidi -l
> > > ALSA lib seq_hw.c:466:(snd_seq_hw_open) open /dev/snd/seq failed: Permission denied
> > > Cannot open sequencer - Permission denied
> > > 
> > > ???
> > 
> > Hrm, that's unexpected.
> > 
> > Meanwhile, I reconsidered the fix and a better idea came after the
> > sleep.  Below is the new fix patch.  Could you give it a try instead
> > of the previous one?
> > 
> > 
> > thanks,
> > 
> > Takashi
> > 
> > -- 8< --
> > From: Takashi Iwai <tiwai@xxxxxxx>
> > Subject: [PATCH] ALSA: seq: Fix racy deletion of subscriber
> > 
> > It turned out that the current implementation of the port subscription
> > is racy.  The subscription contains two linked lists, and we have to
> > add to or delete from both lists.  Since both connection and
> > disconnection procedures perform the same order for those two lists
> > (i.e. src list, then dest list), when a deletion happens during a
> > connection procedure, the src list may be deleted before the dest list
> > addition completes, and this may lead to a use-after-free or an Oops,
> > even though the access to both lists are protected via mutex.
> > 
> > The simple workaround for this race is to change the access order for
> > the disconnection, namely, dest list, then src list.  This assures
> > that the connection has been established when disconnecting, and also
> > the concurrent deletion can be avoided.
> > 
> > Reported-by: folkert <folkert@xxxxxxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Link: https://lore.kernel.org/r/20210801182754.GP890690@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> > ---
> >  sound/core/seq/seq_ports.c | 39 ++++++++++++++++++++++++++------------
> >  1 file changed, 27 insertions(+), 12 deletions(-)
> > 
> > diff --git a/sound/core/seq/seq_ports.c b/sound/core/seq/seq_ports.c
> > index b9c2ce2b8d5a..84d78630463e 100644
> > --- a/sound/core/seq/seq_ports.c
> > +++ b/sound/core/seq/seq_ports.c
> > @@ -514,10 +514,11 @@ static int check_and_subscribe_port(struct snd_seq_client *client,
> >  	return err;
> >  }
> >  
> > -static void delete_and_unsubscribe_port(struct snd_seq_client *client,
> > -					struct snd_seq_client_port *port,
> > -					struct snd_seq_subscribers *subs,
> > -					bool is_src, bool ack)
> > +/* called with grp->list_mutex held */
> > +static void __delete_and_unsubscribe_port(struct snd_seq_client *client,
> > +					  struct snd_seq_client_port *port,
> > +					  struct snd_seq_subscribers *subs,
> > +					  bool is_src, bool ack)
> >  {
> >  	struct snd_seq_port_subs_info *grp;
> >  	struct list_head *list;
> > @@ -525,7 +526,6 @@ static void delete_and_unsubscribe_port(struct snd_seq_client *client,
> >  
> >  	grp = is_src ? &port->c_src : &port->c_dest;
> >  	list = is_src ? &subs->src_list : &subs->dest_list;
> > -	down_write(&grp->list_mutex);
> >  	write_lock_irq(&grp->list_lock);
> >  	empty = list_empty(list);
> >  	if (!empty)
> > @@ -535,6 +535,18 @@ static void delete_and_unsubscribe_port(struct snd_seq_client *client,
> >  
> >  	if (!empty)
> >  		unsubscribe_port(client, port, grp, &subs->info, ack);
> > +}
> > +
> > +static void delete_and_unsubscribe_port(struct snd_seq_client *client,
> > +					struct snd_seq_client_port *port,
> > +					struct snd_seq_subscribers *subs,
> > +					bool is_src, bool ack)
> > +{
> > +	struct snd_seq_port_subs_info *grp;
> > +
> > +	grp = is_src ? &port->c_src : &port->c_dest;
> > +	down_write(&grp->list_mutex);
> > +	__delete_and_unsubscribe_port(client, port, subs, is_src, ack);
> >  	up_write(&grp->list_mutex);
> >  }
> >  
> > @@ -590,27 +602,30 @@ int snd_seq_port_disconnect(struct snd_seq_client *connector,
> >  			    struct snd_seq_client_port *dest_port,
> >  			    struct snd_seq_port_subscribe *info)
> >  {
> > -	struct snd_seq_port_subs_info *src = &src_port->c_src;
> > +	struct snd_seq_port_subs_info *dest = &dest_port->c_dest;
> >  	struct snd_seq_subscribers *subs;
> >  	int err = -ENOENT;
> >  
> > -	down_write(&src->list_mutex);
> > +	/* always start from deleting the dest port for avoiding concurrent
> > +	 * deletions
> > +	 */
> > +	down_write(&dest->list_mutex);
> >  	/* look for the connection */
> > -	list_for_each_entry(subs, &src->list_head, src_list) {
> > +	list_for_each_entry(subs, &dest->list_head, dest_list) {
> >  		if (match_subs_info(info, &subs->info)) {
> > -			atomic_dec(&subs->ref_count); /* mark as not ready */
> > +			__delete_and_unsubscribe_port(dest_client, dest_port,
> > +						      subs, false,
> > +						      connector->number != dest_client->number);
> >  			err = 0;
> >  			break;
> >  		}
> >  	}
> > -	up_write(&src->list_mutex);
> > +	up_write(&dest->list_mutex);
> >  	if (err < 0)
> >  		return err;
> >  
> >  	delete_and_unsubscribe_port(src_client, src_port, subs, true,
> >  				    connector->number != src_client->number);
> > -	delete_and_unsubscribe_port(dest_client, dest_port, subs, false,
> > -				    connector->number != dest_client->number);
> >  	kfree(subs);
> >  	return 0;
> >  }
> > -- 
> > 2.26.2
> 
> 
> Folkert van Heusden
> 
> -- 
> MultiTail is a versatile tool for watching logfiles and output of
> commands. Filtering, coloring, merging, diff-view, etc.
> http://www.vanheusden.com/multitail/
> ----------------------------------------------------------------------
> Phone: +31-6-41278122, PGP-key: 1F28D8AE, www.vanheusden.com
> 



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